Narrow band antenna.

[Martin Griffith]

Yes...very good. That should help keep the loop nicely balanced,

Hi. I have some problem. I can't receive nothing except noise And self
oscillation frequency.
I have some photos. Could any give me some suggestions?
Antenna:
http://artembond.no-ip.info/apache2-default/DSC_9427.JPG

Chematics:
http://artembond.no-ip.info/apache2-default/DSC_9431.JPG

Amplifier:
http://artembond.no-ip.info/apache2-default/DSC_9426.JPG

I think you need to narrow band those photos.

greg[/QUOTE]

Yes
www.irfanview.com has an excellent bit of software that does just that


martin

 

[Joerg]

Citation would be an article written by an engineer at Andrew and
published 10-15 years ago that I unfortunately can't find.
"Polishing" (implying a mirror-like finish) certainly isn't necessary,
but I do think it's worthwhile getting it reasonably clean and then
protecting it so corrosion doesn't set in. In the case of the antenna
presented here, the copper resistance is about ten times the radiation
resistance, so there is some incentive to keeping it as low as
practical. In the case of a typical resonant dipole, the copper
resistance is such a small percentage of the radiation resistance that
it's much less concern.

I remember the pain in my wrists from polishing load coils and such.
Yes, it did make a difference and if you slacked off for too long the
stuff would actually become hot when exposed to a few hundred watts.

So, I learned to develop enough dexterity in my left hand in order to
switch wrists from time to time. Many tubes of Wenol polishing paste
have gone into this effort over the years.

 

[Richard Clark]

Hi. I have some problem. I can't receive nothing except noise And self
oscillation frequency.

Try choking both what are labeled "RF Out" and "Shield Cable."

73's
Richard Clark, KB7QHC

 

[K7ITM]

Yes...very good. That should help keep the loop nicely balanced,


Hi. I have some problem. I can't receive nothing except noise And self
oscillation frequency.
I have some photos. Could any give me some suggestions?
Antenna:http://artembond.no-ip.info/apache2-default/DSC_9427.JPG

Chematics:http://artembond.no-ip.info/apache2-default/DSC_9431.JPG

Amplifier:http://artembond.no-ip.info/apache2-default/DSC_9426.JPG

I saw the comment about "narrow-banding" the images. They were
perhaps a little more than we needed, but it was nice to have
something we could actually see. They did not take very long to
download here, but someone with a slow connection may have troubles.

One comment: usually you do not need much voltage gain. It is enough
to get power gain with the FETs. That is, the received signal voltage
across the gap of the loop, as resonated by the capacitors, should be
high enough to be used with a good receiver. The problem is that the
impedance is very high there. But that same high impedance makes for
easy oscillation. From what you posted, it sounds like maybe you have
identified an oscillation. If the AGC voltage is low enough, does the
oscillation stop? The amplifier I built used two stages, an FET input
stage and a buffer stage, and it had very low voltage gain--I am
remembering about 3:1 or only 10dB, and maybe only 1:1 or 0dB
including the output transformer, but quite a bit of power gain since
it transformed the high loop impedance down to 50 ohms for the
feedline.

Also, there should be no need for the RF chokes from the gate-1 to
source, if the loop is grounded at the bottom. If the loop is
grounded at the bottom, the loop plus the RF chokes will short out the
source-to-ground resistor. Maybe there is not a need to raise the
source voltage above DC ground potential anyway. Also, it may help to
NOT bypass the sources to ground, to allow some negative feedback.
That may help stabilize the amplifier.

If other things occur to me, I will post them...

Cheers,
Tom

 

[JosephKK]

I remember the pain in my wrists from polishing load coils and such.
Yes, it did make a difference and if you slacked off for too long the
stuff would actually become hot when exposed to a few hundred watts.

So, I learned to develop enough dexterity in my left hand in order to
switch wrists from time to time. Many tubes of Wenol polishing paste
have gone into this effort over the years.

Hunh? WTH? I have had loading coils, tuning coils, etc. go for
years without polishing at 1 KW in regular use. An HF transceiver to
be sure, but what is the difference here?

 

[Artem]

I saw the comment about "narrow-banding" the images. They were
perhaps a little more than we needed, but it was nice to have
something we could actually see. They did not take very long to
download here, but someone with a slow connection may have troubles.

I'm move images to ImageShack
http://img524.imageshack.us/my.php?image=dsc9427ib7.jpg
http://img262.imageshack.us/my.php?image=dsc9431tv9.jpg

One comment: usually you do not need much voltage gain. It is enough
to get power gain with the FETs. That is, the received signal voltage
across the gap of the loop, as resonated by the capacitors, should be
high enough to be used with a good receiver. The problem is that the
impedance is very high there. But that same high impedance makes for
easy oscillation. From what you posted, it sounds like maybe you have
identified an oscillation. If the AGC voltage is low enough, does the
oscillation stop?

Yes. It's stop. my main reason for this antenna is make very narrow
antenna for city. I can't receive to my receiver almost nothing by big
nose from computers, lamps and other things. But looks like this
antenna did not help. Its amplify narrow-band noise. And even add more
noise when oscillated.

The amplifier I built used two stages, an FET input

stage and a buffer stage, and it had very low voltage gain--I am
remembering about 3:1 or only 10dB, and maybe only 1:1 or 0dB
including the output transformer, but quite a bit of power gain since
it transformed the high loop impedance down to 50 ohms for the
feedline.

Also, there should be no need for the RF chokes from the gate-1 to
source, if the loop is grounded at the bottom.

Source is not grounded for DC. For better transistors matching and
overcurrent protection.

If the loop is

grounded at the bottom, the loop plus the RF chokes will short out the
source-to-ground resistor. Maybe there is not a need to raise the
source voltage above DC ground potential anyway. Also, it may help to
NOT bypass the sources to ground, to allow some negative feedback.
That may help stabilize the amplifier.

Sources is not grounded.

 

[Artem]

Try choking both what are labeled "RF Out" and "Shield Cable."

Thank. It's good idea. I'll try.

 

[[email protected]]

I remember the pain in my wrists from polishing load coils and such.
Yes, it did make a difference and if you slacked off for too long the
stuff would actually become hot when exposed to a few hundred watts.

So, I learned to develop enough dexterity in my left hand in order to
switch wrists from time to time. Many tubes of Wenol polishing paste
have gone into this effort over the years.

Can you use a cotton strip looped around the conductor and saw it back
and forth?

Called "ragging" very common in musical instrument business going back
hundreds of years.

 

[K7ITM]

I'm move images to ImageShackhttp://img524.imageshack.us/my.php?image=dsc9427ib7.jpghttp://img262.imageshack.us/my.php?image=dsc9431tv9.jpg




Yes. It's stop. my main reason for this antenna is make very narrow
antenna for city. I can't receive to my receiver almost nothing by big
nose from computers, lamps and other things. But looks like this
antenna did not help. Its amplify narrow-band noise. And even add more
noise when oscillated.

The amplifier I built used two stages, an FET input



Source is not grounded for DC. For better transistors matching and
overcurrent protection.

If the loop is> grounded at the bottom, the loop plus the RF chokes will short out the

Sources is not grounded.

Yes, unfortunately noise that is generated more than one or two
wavelengths away from your antenna will be almost entirely
electromagnetic by the time it reaches your antenna. Antennas do not
differentiate between "electrically generated" and "magnetically
generated" noise, when you are far enough that the electromagnetic
field dominates over any near-field electric or magnetic field. The
balanced small loop is good for rejecting electric-field noise only if
(1) the noise is generated close to the antenna and (2) the antenna is
close to the ground (so the electric field is guaranteed to be nearly
vertical) -- -- where "close" means relative to a wavelength. So the
small balanced loop is especially good for LF and VLF work.

Perhaps someone else will have suggestions about what else you might
try.

Cheers,
Tom

 

[Artem]

Yes, unfortunately noise that is generated more than one or two
wavelengths away from your antenna will be almost entirely
electromagnetic by the time it reaches your antenna. Antennas do not
differentiate between "electrically generated" and "magnetically
generated" noise, when you are far enough that the electromagnetic

I did not hear nothing about electrically or magnetically photons.
It's just photons.

field dominates over any near-field electric or magnetic field. The
balanced small loop is good for rejecting electric-field noise only if
(1) the noise is generated close to the antenna and

Yes. Computer, lamps etc close to antenna.

(2) the antenna is
close to the ground (so the electric field is guaranteed to be nearly

15 floor of 16-floor building. But I think that in this case "ground"
are building walls.

vertical) -- -- where "close" means relative to a wavelength. So the
small balanced loop is especially good for LF and VLF work.

my reason was make narrow-band antenna. For reject all out of band
noise.

 

[Artem]

I remember the pain in my wrists from polishing load coils and such.

Yes, it did make a difference and if you slacked off for too long the
stuff would actually become hot when exposed to a few hundred watts.

http://img86.imageshack.us/my.php?image=polishvt4.jpg

 

[John Ferrell]

my reason was make narrow-band antenna. For reject all out of band
noise.

That is a very worthwhile objective. All that noise adds to the
intermod noise and the sooner you can stop it, the better.

In my youth I recall calling CQ on 42 mhz for a couple of weeks.
Fortunately, a very sharp yagi and very low power saved me from a
violation.

John Ferrell W8CCW
Beware of the dopeler effect (pronounced dope-ler).
That's where bad ideas seem good if they come at you
fast enough.

 

[K7ITM]

I did not hear nothing about electrically or magnetically photons.
It's just photons.

Oh, no, not photons again! When you are near to a source -- to a
transmitting antenna or to a computer radiating noise -- the fields in
general have not developed fully into electromagnetic waves -- photons
if you wish. It is quite usual that, close to the source, either the
electric or the magnetic field will dominate. Often from noise
sources, the near field is predominantly electric, and a properly done
loop antenna will reject that, responding only to the, um, photons.

Yes. Computer, lamps etc close to antenna.


15 floor of 16-floor building. But I think that in this case "ground"
are building walls.

There is a hint here: it is common that tall buildings incorporate a
lot of steel, and that will likely act as a shield. I hope this
antenna is not mounted inside!

my reason was make narrow-band antenna. For reject all out of band
noise.

A reasonable thing to do, though a good receiver with a low-distortion
and fairly narrow-band front end should not have trouble with out-of-
band signals (noise). Do you have a quantitative measure of just how
strong this out of band noise is? I'd personally much rather use a
preselection filter separate from the antenna, and close to my
operating position, to reject out-of-band signals. Even though the
antenna you have described has very high Q, I believe I could do
better with a two or three resonator filter running at lower Q, since
the slope of the attenuation versus frequency is much greater. Unless
there was some especially strong signal in the band, I would at least
consider a fixed-tuned bandpass filter that covered my band of
interest, assuming that band is fairly narrow such as 7.0-7.1MHz.

Can you tell that you are getting the expected antenna bandwidth,
about 3kHz at the 3dB points at 7MHz?

If the amplifier at the antenna has a tendency to oscillate, it very
likely also has poor intermodulation performance. Be careful that it
doesn't destroy the benefits you are trying to obtain.

Cheers,
Tom

 

[Artem]

There is a hint here: it is common that tall buildings incorporate a
lot of steel, and that will likely act as a shield. I hope this
antenna is not mounted inside!

It's not mounted at all. But for tests I'm put this antenna outside.

A reasonable thing to do, though a good receiver with a low-distortion
and fairly narrow-band front end should not have trouble with out-of-
band signals (noise). Do you have a quantitative measure of just how
strong this out of band noise is?

Not. Just not received.

I'd personally much rather use a

preselection filter separate from the antenna, and close to my
operating position, to reject out-of-band signals. Even though the
antenna you have described has very high Q, I believe I could do
better with a two or three resonator filter running at lower Q, since
the slope of the attenuation versus frequency is much greater.

I will receive QRSS at all. And I think that it would be best way is
using
narrow-band antenna -> filter -> synchronous detector.

there was some especially strong signal in the band, I would at least
consider a fixed-tuned bandpass filter that covered my band of
interest, assuming that band is fairly narrow such as 7.0-7.1MHz.

Can you tell that you are getting the expected antenna bandwidth,
about 3kHz at the 3dB points at 7MHz?

I'm just testing. I will purchase RF generator in next week and test.
Now I have only self-oscillation frequency.

Antenna looks like working. I'm receiving a lots of Morse signals at
7.000 - 7050 Mhz. But I cant recognize any voice signal.

This is receiving signal. Looks like narrow-band enough. This is not
self oscillation. In self oscillation voltage a few volts.
http://img148.imageshack.us/my.php?image=ds0000bu6.png

This is schematics. I'm not sure that I'm correct use gual gate
transistors.
http://img210.imageshack.us/my.php?image=schbr1.jpg

I'm not sure that using shielded cable and ferrite chocks is good
idea.
http://img171.imageshack.us/my.php?image=hwak2.jpg

np0 caps.
http://img370.imageshack.us/my.php?image=capsnf8.jpg

 

[Artem]

COOL! All worked!

 

[Artem]

Side Band?

No. Out of band. I'm now add varicaps and all working! Antenna really
very narrow.

Not enough choking and needs to be repeated a quarter wave down the
choked wire(s).

It's 7Mhz. quarter wave is 10 miters.

I'm use choking:
between amplifier and antenna
between transformer and coax cable.
For amplifier power wires and gain control.

 

[Richard Clark]

Antenna looks like working. I'm receiving a lots of Morse signals at
7.000 - 7050 Mhz. But I cant recognize any voice signal.

Side Band?

I'm not sure that using shielded cable and ferrite chocks is good
idea.

Not enough choking and needs to be repeated a quarter wave down the
choked wire(s).

73's
Richard Clark, KB7QHC

 

[Richard Clark]

It's 7Mhz. quarter wave is 10 miters.

Then again at the far end of the wire(s).

I'm use choking:
between amplifier and antenna
between transformer and coax cable.
For amplifier power wires and gain control.

I've observed that, and I have observed it is not enough from your
photo - if you still have self-oscillation. Your pictures do not
reveal any choking of the RF Out cable.

As for the diagonal arm for "ground." This is fine insofar as it
being placed in the electrical middle of the antenna loop (a ground),
but all this rat's nest of wiring throws the concept of balance out
the window. I see in earlier pictures where you clean that up with
careful routing, but it seems much too busy. This leads me into the
comments following:

On another note, the AGC seems overly elaborate, especially when all
that wire could be introducing the self-oscillation. Further, wiring
in power seems another invitation to problems when a 9V battery would
solve that too. Local power would discard the need for the ground
coming from the loop's perimeter, eliminate unnecessary AGC, reduce
the complexity of choking, lower gain (it obviously has too much), and
give you only one coax coming from the antenna.

You need to solve the self-oscillation through removing complexity.
When you accomplish that, THEN that is the time to add it back in, one
thing at a time. You will probably discover all those features and
design gimmicks are not worth it.

73's
Richard Clark, KB7QHC

 

[Richard Clark]

COOL! All worked!

It would seem no further advice is necessary, much less my last bit of
wisdom on the topic.

It would help if you were to elaborate as to what actually killed the
oscillation.

73's
Richard Clark, KB7QHC

 

[Artem]

Then again at the far end of the wire(s).


I've observed that, and I have observed it is not enough from your
photo - if you still have self-oscillation. Your pictures do not
reveal any choking of the RF Out cable.

It's inside. Nearby BNC socket.

As for the diagonal arm for "ground." This is fine insofar as it
being placed in the electrical middle of the antenna loop (a ground),
but all this rat's nest of wiring throws the concept of balance out
the window.

I think that some disbalance should compensate differencial amplifier
on transistors.

I see in earlier pictures where you clean that up with
careful routing, but it seems much too busy. This leads me into the
comments following:

On another note, the AGC seems overly elaborate, especially when all
that wire could be introducing the self-oscillation. Further, wiring
in power seems another invitation to problems when a 9V battery would
solve that too. Local power would discard the need for the ground

Yes. But FETs draw more that 10ma each.

coming from the loop's perimeter, eliminate unnecessary AGC, reduce
the complexity of choking, lower gain (it obviously has too much), and
give you only one coax coming from the antenna.

Cable length is not problem. I'm living in apartment. I can put
antenna outside the window. But not on the roof.

I can make power supply over coax cable. I can put Atmega8 (en
example) to amplifier and add DACs for operate varicaps, AGC. I can
add rectifier and filter for detect self-oscillation and automatics
reduce AGC. But it's not necessary.